In seismic exploration it is known to generate seismic pulses or waves from at least one seismic source and to measure or record the wavefield using a plurality of seismic receivers. Seismic sources are either of the impulse type generating a sharp and sudden peak of wave energy or, alternatively, of the vibrating type generating a sweeping signal of ideally controlled amplitude and frequency spectrum. Marine seismic sources commonly used are impulsive sources comprising a plurality of so-called “airguns” as source elements arranged in an array to produce a combined seismic source which has more desirable characteristics than the individual source elements of the array. Marine vibratory sources exist but are less frequently used.
In use, an airgun generates a high pressure air bubble by the sudden discharge of a quantity of high pressure compressed air into the water. According to established theoretical knowledge, the elasticity of the air couples with the inertial mass of the surrounding water to produce an oscillating system as the air expands and contracts in size until its energy is dissipated in the water and the bubble reaches its equilibrium volume. These bubble oscillations generate spherical sound waves which form the seismic signal. As described below in further detail, it is known that marine seismic signals can be synchronized so as to enhance the primary pulse in an acquisition method referred to as peak-tuning, or, if the synchronization is tuned to the first bubble, bubble-tuning. The synchronization may alternatively be tuned to any other part of the composite signature of the source.
In is an important, but not necessarily desirable, feature of an array of marine seismic source elements, which form a seismic source, that the sound wave transmitted through the body of water is directional, i.e. the shape or signature of the transmitted wave varies with vertical polar angle, and azimuthal polar angle for a source not designed to be azimuthally symmetric (such special sources being described for example in United Kingdom patent GB 2376528). This is seen as a result of (i) the array having dimensions which are not negligible compared to the wavelengths of sound in the transmitted wave and (ii) the effect of the free-surface ghost reflection causing each source element to have an approximately equal and opposite virtual image source element in the free-surface mirror when observed at distances far from the source. In a given direction, the signature of a transmitted wave varies in the so-called “near field” as the distance from the array increases until at a sufficient distance from the array, in the so-called “far field”, the shape of the wave remains substantially constant but the amplitude decreases, generally inversely in proportion to the distance from the array. The “far field” of an array or source generally exists at distances greater than D2/λ where D is the dimension of the array and λ is the wavelength.
In U.S. Pat. No. 4,476,553 and in the European Patent EP 0066423, there is disclosed the use of an array of near-field hydrophones or pressure sensors arranged to measure the seismic signal generated by an array of airgun elements in a body of water. Each hydrophone is placed no closer than about 1 m to an associated airgun so that the pressure measured at each hydrophone is a linear superposition of the spherical waves from all the oscillating bubbles and their reflections in the free surface. Using the hydrophone signals a synthetic source signal can be derived, the “notional source”, which provides an efficient way of determining the far-field signature of the source in all angular directions.
A variant of the above known source is the TRISOR™ source used by WesternGeco Ltd. The TRISOR marine source controller enables the airgun elements to be synchronized so as to enhance the primary pulse (peak-tuning), or the first bubble (bubble-tuning) or any other part of the composite airgun signature. TRISOR also allows acquisition of data from a hydrophone located near to each airgun element. Although commonly referred to as near-field hydrophones (NFH), the trace from each hydrophone is actually in the far-field of the acoustic pressure radiated from the airgun.
In TRISOR the notional source algorithm as described in: Ziolkowski, A., Parkes, G., Hatton, L. and Haugland, T., The signature of an air-gun array—Computation from near-field measurements including interactions. Geophysics 47, 1413-1421 (1982) and in the European Patent EP 0066423 can be used to compute far-field signatures of the array as a whole directly below the marine source array, or for any take-off direction in the 2π steradians centered upon the vertical line below the acoustic centre of the array and characterized by vertical polar and azimuthal polar angles. Far-field in this context means a distance which is large compared to the scale length of the marine source array, typically 10-20 m, or its depth of immersion, typically 5-20 m, so that while the composite signature shape is independent of distance, it may still vary with direction.
U.S. Pat. No. 5,247,486 describes a method for determining a far-field signature of a plurality of seismic source elements measuring a near-field signature of each seismic source element. In a preliminary stage an initial near-field signature of each seismic source element and an initial far-field signature of the plurality of N seismic source elements are measured simultaneously. An operator is determined to calculate subsequent far-field signatures.
WO-2004068170-A1 provides a method and apparatus for directional de-signature of a seismic signal. The method includes forming a plurality of far-field signatures representative of a plurality of seismic signals having a plurality of take-off angles, associating a plurality of traces representative of a plurality of reflections of the seismic signals with the plurality of far-field signatures, and forming a plurality of de-signatured traces from the plurality of traces and the plurality of associated far-field signatures.
EP-A-0,400,769 discloses an array of air guns with near-field hydrophones mounted 10 to 15 cm from the gun ports (i.e. in the non-linear zone) and EP-A-0,555,148 discloses a method of predicting wave signatures.
A known seismic source airgun made by Bolt Technology Corporation is disclosed in U.S. Pat. No. 4,240,518 and incorporates a stainless steel pressure sensor, known as the BSS, which is mounted within the airgun to measure the pressure inside the gun. With an array of airguns making up a seismic source it is important to ensure that all the guns fire at the same time and the signals from the pressure sensors on the different guns are used not only to measure the firing instant, or “time-break”, of each gun but are also used as input to the gun controller, which adjusts the timing of the firing commands to the individual guns.
Further details of the techniques and advantages of bubble-tuning are described by Avedik, F., Renard, V., Allenou, J. P., and Morvan, B., Single bubble air-gun array for deep exploration. Geophysics 58, 366-382 (1993) and in the French patent application FR-A-2,702,281. A further study on the topic of bubble-tuning can be found in: Lunnon, Z., Christie, P., and White, R., (2003). An evaluation of peak and bubble tuning in sub-basalt seismology: modelling and results from OBS data. First Break, 21(12), 51-56(2003).
An optimal deconvolution filter using semblance-weighted deconvolution is described in: Haldorsen, J., Miller, D. and Walsh, J. Multichannel Wiener deconvolution of vertical seismic profiles: Geophysics, Soc. of Expl. Geophys., 59, 1500-1511(1994). The method is used for estimating an optimal signature from a plurality of depth-dependant source signatures.
Because the path length from the marine source through the earth to the receivers is also large compared to the array dimension, the source is often approximated as a point and the vertically-downward, far-field signature is commonly taken as the signature for shaping the seismic data either to minimum phase or sometimes to zero phase. In fact, far-field signatures vary with the vertical and azimuthal polar angles from the array centre but for the majority of conventional surveys, utilizing peak-tuned sources and vertical polar angles below 30°, the effect is small. However, in Christie, P., Langridge, A., White, R., Lunnon, Z., Roberts, A. W. and the iSIMM team, (2004). “iSIMM looks beneath basalt for both industry and university research”, Ext. Abs. 87 presented at 5th Petroleum Geophysics Conference, Hyderabad, India, a deep-towed source is tuned on the first bubble to provide a signature rich in low frequencies for sub-basalt penetration. While a high quality section has resulted from a processing flow based upon sub-critical offsets, it is observed that bubble-tuned signatures can vary more rapidly with the vertical polar angle than peak-tuned signatures. The present invention is partly motivated by this observation.
The strong variation of the bubble-tuned signatures with the vertical polar angle is a problem in source signature deconvolution of recorded seismic data when debubbling the mixed-phase bubble-tuned signature and shaping it to zero phase. Significant pre-cursor energy can be created by the deconvolution causing noise at higher offsets. The pre-cursor energy results from the variation with angle of the notch depths and frequencies of the bubble-tuned signature. After deconvolution by an operator designed as known from the vertically-downgoing signature, the spectral whitening amplifies energy in the signature spectra at higher angles.
Whilst it is possible to use an angular-dependent deconvolution to improve the process (e.g.: van der Schans, C. and Ziolkowski, A. M., 1983, Angular-dependent signature deconvolution, 53rd Ann. Internat. Mtg: Soc. of Expl. Geophys., Session:S13.3), the amount of additional processing required makes this approach currently not economically viable. The present invention, therefore, seeks to provide an efficient method for the source signature deconvolution of recorded seismic data, the method being applicable to a wider range of polar or take-off angles.